The present invention relates to an internal gear pump and, more particularly, to a small-sized internal gear pump. A conventional internal gear pump of a first type comprises a casing, a cover mounted to the casing, a pump chamber defined between the casing and the cover when assembled with each other, an internal gear slidably arranged within the pump chamber, an external gear rotatably arranged within the pump chamber in meshing relation with the internal gear and rotatingly driven from the outside to drive the internal gear for carrying out a pumping action, a fixed filler for regulating sliding movements of the internal and external gears and for preventing oil leakage from a high-pressure side to a lower-pressure side within the pump chamber, first flat-bottomed oil pockets formed in the casing, one of which is provided in a sliding surface of the casing to face a suction port formed in the cover and is extended over about one-fourth the circumference of a circle on the casing, and the other of which is provided in the sliding surface of the casing to communicate with a discharge port of the casing and is extended over about one-fourth the circumference of a circle on the casing, and another first flat-bottomed oil pocket provided in the cover to face the first oil pocket of the casing communicated to the discharge port and extended over about one-fourth the circumference of a circle on the cover.
A conventional internal gear pump of a second type disclosed in Japanese Patent Unexamined Publication No 55-152209 is different from the above internal gear pump of the first type in that although there are provided third flat-bottomed oil pockets in those surfaces of a casing and a cover, on which an external gear slides, said oil pockets being extended over about one-fourth the circumference of a circle on the casing and on the cover, there are not any second oil pockets at those portions of the casing and the cover which face the opposite sides of the internal gear adapted to slide between the outer peripheral wall of a fixed filler and the inner peripheral wall of the casing. As described above, the oil pockets in the first and second types of the internal gear pump are flat-bottomed in configuration. In the prior internal gear pump described above, the sliding surfaces of the casing and the cover, on which the both gears slide, have a high level of sliding resistance, so that the internal gear pump is reduced in the total efficiency and also in the volumetric efficiency and become high in temperature at the sliding surfaces of the casing and the cover.